In a submerged pump of the prior art which is provided with a pump casing outside of the motor for driving the pump, it has been generally known to form the pump casing in a cylindrical configuration so as to provide a substantially uniform gap between the casing and the motor. Consequently, water drawn from a suction port of the pump and passed through volutes of the pump is advanced toward a discharge opening through the gap wherein the flow of water follows a spiral path. Further, at least two volutes are normally provided in a volute casing of the pump so as to balance radial thrust. Thus, each water flow discharged from an outlet of the respective volute will interfere with other flow during the passage of the gap where the respective flow takes the spiral path.
The spiral passage causes the water flow to stay longer in the gap and to increase the friction loss. Also, due to the interference between the water flows, several other losses will be derived thereby remarkably reducing the efficiency of the pump.